From my understanding the iOS versions of AutoCad are in no way meant to serve as a replacement for the desktop version. But rather a very simplified version of the tools and features available.
As a designer, I can imagine being out for coffee or dinner with a potential client and not having my Macbook with me. Being able to pull out my iPhone or iPad and do a quick and dirty sketch of my thoughts/ideas to better convey the message could be incredibly useful. Much better than a standard cocktail napkin.
AutoCad is seeking to simply fill a void in a complete cross-hardware line. This addition will enable users to "CAD" wherever and whenever within certain constraints. These applications will work seamlessly together - so a quick sketch on your iPhone can later be redrawn and upgraded on your desktop.